Contributing.md: Building the Documentation needs Mac-specific step

[jkernion-adi]

The Building the Documentation section of Contributing.md contains some instructions that are Windows-specific and do not have an intuitive alternative for Mac users.

Specifically, under Local builds, "To build the docs": step 2 links to Windows instructions for Environment Path variable. Would be helpful to have Mac instructions for adding directory to path. (Also, "directory" is misspelled in this step.)